It is well-known that one of the most considerable problems found in hot-rolling lines, particularly in lines for long products in bars, is the need to keep under constant control the drawing action on the rolled stock between an upstream stand and a downstream stand, or between a stand and a drawing assembly or again in the same stand where there are two or more rolling modules or blocks combined.
This requirement is increased when, in a segment between stands, there are heat treatment or conditioning systems which can modify the structural characteristics of the material, upstream and downstream of the systems, in terms of resistance to the drawing action and which therefore make it difficult to control and regulate the drawing action in the inter-stand segment.
If the drawing action is irregular and/or inconstant in time, it will cause an unacceptable deterioration in quality and dimensional irregularities which cause reduced yields, stoppages in the line, a need for frequent adjustments and other problems which negatively affect the operativity and productivity of the line.
Moreover, if the drawing action is not controlled precisely and constantly, it is not possible to make those adjustments to the line which allow the product to be maintained within strict dimensional tolerances in a rigorous and automated manner which can be repeated whatever the conditions upstream may be, without having recourse to complex and intricate adjustments to the gap between the rolls; these adjustments cause stoppages of the line, for the most part they are entrusted to the workers' experience and skill and cause operational pauses which negatively affect the productivity of the line.
In the state of the art various solutions have been proposed to rigorously measure the drawing action and to control in feedback the drawing assemblies in order to maintain controlled and constant drawing conditions.
For example, some solutions have proposed diameter measuring devices located immediately at the outlet of the rolling stand, or at the inlet and outlet of the stand, and governing in feedback the motors of the rolls in the event that there is any divergence with respect to the expected nominal dimension.
Measuring devices have also been proposed which detect the tension of the rolled stock in the segment between stands so as to determine dimensional pulses in the section and thus establish the relative drawing conditions.
These and other solutions of the state of the art have not, however, given a complete and satisfactory solution to the technological and operational need to guarantee a control of the drawing action which is both reliable, simple, economical, practical and extremely accurate.
The present Applicant, in the European Patent application EP-A-0756906, proposed an innovative solution to control the drawing action of the rolled stock, based on at least two dimensional detectors in the segment between the stands: a first detector immediately at the outlet of the upstream stand and a second detector immediately at the inlet of the downstream stand. In the event that there is a heat treatment performed in the inter-stand segment there is also a dimensional detector upstream of the treatment.
This solution indeed gives precise information on the regular condition of the drawing action of the rolled stock, inasmuch as it identifies unacceptable dimensional deformities and reductions in section caused by irregular and inconstant drawing in the inter-stand segment.
Although this embodiment is in itself positive, it does not determine any irregularities in the rolling or drawing which occur inside the stand itself, or between rolling blocks combined in the same block; nor does it identify with precision the reasons which cause the irregularities, in order to intervene with a feedback command so as to restore the correct rolling and drawing conditions.
DE-A-1.452.062 describes a method to adjust the size of steel shapes subjected to rolling.
This document explains how and in what position to use rolling finishing stands which form arcs in the material, which have the function of reducing the surface defects, in terms of size, of the product which has been rolled.
It also explains how the adjustment in size of the rolled stock is carried out by means of two methods, wherein the thickness of the rolled stock is corrected by choosing the position of the stands wherein arcs are formed in the material in the rolling train, whereas the width is corrected by adjusting the number of revolutions of the first command device in the finishing train.
This document therefore includes two systems of measuring the size in order to verify that tolerances are respected at the outlet of the finishing train, a first system which measures the thickness of the rolled stock and a second system which measures the width of the rolled stock.
This document describes a complex system, which requires two distinct measuring systems, each for one size of the rolled stock, connected to two respective adjustment systems which work independently of each other.
This system involves complex operations and very long adjustment times, inasmuch as making a correction to the size, for example to the thickness, can increase the defect of size in the width, and vice versa; this leads to a continual chasing after the correct values by means of successive adjustments.
Moreover, the apparatus is costly, complex to manage and complex in functioning.
Furthermore, this document does not take into account the fact that the round piece emerging at high speed from the finishing block is subject to torsion on its own axis.
Because the section of the round piece is not perfectly circular, this torsion has the result that the measurements made independently of the thickness and the width are completely distorted and cause corrections to be made which are not even necessary, or on the contrary do not recognise values which are outside the tolerance limits because the measurement has not been made in the correct position.
The Applicant has devised and embodied this invention to solve these problems with a global method which will ensure the constant dimensional control of the rolled stock with a simple, economical, reliable and functional solution.